Welsh language campaigners have been placing stickers on English-only road signs which they claim are illegal because they are not bilingual.

Cymdeithas yr Iaith say it is ‘unacceptable’ for Powys County Council to treat the Welsh language with ‘such disregard’.

In Llanidloes, the campaign group’s Montgomeryshire branch – Cell Maldwyn – have placed a sticker on an English-only ‘Give Way’ sign at Westgate Street junction with a Welsh ‘yield’ instruction – ‘Ildiwch’. At New Street junction, a sticker with ‘ble mae’r Gymraeg?’ (Where's the Welsh?) has been stuck on a non-bilingual Great Oak Street sign for heavy goods vehicles.

They say there are non-bilingual signs in Llandinam and Llanerfyl which they have yet to place campaign stickers.

On Twitter, Cell Maldwyn posted pictures of signs with no Welsh translation to Powys County Council’s Welsh-language Twitter account. In Welsh they said: “Why is there an English-only sign in Llanfair Caereinion, Powys County Council? It should say Melin Newydd, Y Drenewydd (New Mills, Newtown). Will you change it, please? If you need help taking it down, we’ve got years of experience!”

The campaign for bilingual road signs – one of the priorities of the Welsh Language Society from the 1960s onwards – was a success, as bilingual signs are now seen throughout Wales.

Around one fifth of the population of Powys (18 per cent) say they can speak Welsh, according to the 2011 census, which is a percentage lower than the national average.

David Williams, a local member of Cymdeithas yr Iaith, said: “For around a quarter of a century Powys Council has been under a legal duty to ensure signage is in Welsh – initially under its own language scheme and now under the Welsh Language Standards system. So, the council has broken the law by erecting these English-only signs.

“In the light of the council’s illegal behaviour, our members are quite right to take it into their own hands to correct them. It is, after all, unacceptable for the council to treat the Welsh language with such disregard.”

A Powys County Council spokesperson said: “The signs referred to are old and when they become unserviceable they will be replaced with bilingual signage. We are not able to replace existing signs which are in good condition due to budget pressures.

“We are concerned that the stickers could impact road safety, especially for the give way sign, as well as blocking important information for motorists.”